I also have problems with BX750MI reporting battery fault condition, maybe some incompatibility with the BX line up.
Brought 2 new units in spring sale, 1 is connected to QNAP464, Other to new Unraid Server
QNAP seems happy with it and reports no issues 100% battery, Unraid flagging its one multiple times a day and shows 99% battery.
When I finally finish transferring all my data I might swap them over see where the fault follows.